1. Field of the Invention
The invention pertains to a method for mounting a shadow mask of the flat, perforated type (known as a flat tension mask) in a trichromatic cathode tube. It pertains especially to a method for the mechanical tensioning of the mask so that it preserves its planarity while functioning. The invention also pertains to a trichromatic cathode tube comprising a shadow mask that is mounted according to this method.
2. Description of the Prior Art
At present, color television manufacturers are directing their production efforts so as to obtain images on surfaces that are as flat as possible, i.e. their aim is that the front panel or envelope of the color television picture tube should be as flat as possible. Manufacturers are capable of producing glass front envelopes that are entirely flat (both externally and internally) and of optimizing the geometry of these envelopes with a view to providing, in particular, protection against implosion. Thus, the main limitations on the use of flat screens are related to the requirements of mounting the shadow mask which, in this case, should also be flat.
The shadow mask constitutes a color selection electrode which, until quite recently, was curved or parabolic in order to obtain an image on an envelope which was also curved. A trichromatic cathode ray tube or color television tube generally comprises a glass casing consisting of a rectangular-shaped front panel or front envelope, often extended by a skirt-shaped lateral wall. The skirt is sealed to a conical part which is narrowed and ends in a tubular or cylindrical neck. The said neck houses a set of three guns at its end. It has electromagnetic deflectors tightly fitted on to its exterior. These deflectors are used to scan a trichromatic luminescent screen. The screen consists of luminophors of three primary colors, red, blue and green, which are deposited on the internal surface of the front envelope. The luminophors are made up of either dots or vertical lines arranged, for example, in a repeated succession of three strips of vertical luminophors of different colors (red, green and blue). The selection of the colors is obtained by a selection electrode known as a shadow mask which is placed on the path of the electron beams which have to bombard the screen. The shadow mask consists of a metallic surface having a shape similar to that of the screen, which is usually domed. Most often, the shadow mask is of the perforated type, namely its surface is pierced with a large number of holes (for example, oblong or rectangular holes), the purpose of which is to let through, for each electron beam, only that part of the beam which will bombard the line or luminophor of the color assigned to this beam.
The curvature of the shadow mask is generally obtained by mechanical shaping operations which increase its mechanical resistance and make it possible to mount it easily, by welding, on a frame which is also domed. The domed shadow mask and the frame constitute an assembly that has great mechanical rigidity making it compatible with batch manufacturing conditions and capable of withstanding handling operations as well as shocks and vibrations.
During the manufacture of the tube, the shadow mask/frame assembly has to be lifted and repositioned several times, especially to make the trichromatic screen.
It must be noted that the perforated mask, and especially its active surface which has the perforations, dissipates a major part of the power of the electronic beams by Joule effect. The result of this is an expansion of the perforated mask which may result in a doming of the mask that modifies the initial alignment between certain perforations of the mask and the luminophors. The result of this is either a decrease in the light intensity proportionate to the surface of the bombarded luminophors or color purity faults. These faults are reduced by the use of a perforated mask with a radius of curvature that is smaller than that of the screen according to method known as the super arched mask method. The expansion of the perforated mask constitutes a limit on the power density (W/cm2) which can be applied by scanning frames.
As compared with a curved screen working with a shadow mask that is also curved, the use of an perforated type of flat mask (known as a flat tension mask or an FTM) provides numerous advantages, for example:
Power density of more than 100 mW/cm2 for a full scanning frame, i.e. about eight times greater than a curved perforated mask;
The possibility of using a perfectly flat screen for both 90.degree. and 110.degree. deflections;
The possibility of being used in a very wide range of applications and in every size, especially for high-definition color picture tubes, possibly for special military applications.
The only possibility of using a flat tension mask is that it should be mounted on a relatively solid frame putting it under adequate mechanical tension so that, during operation, its heating under the bombardment effect of the electron beams does not destroy its planarity.
An approach of this type has been used within the framework of a color-tube manufacturing method which is appreciably different from usual manufacturing methods. In this method, a screen (not flat) with the shape of a cylindrical portion is coupled to a mask known as a grid mask and the holes are replaced by heightwise vertical slits on the screen. Metallic strips forming this mask are mounted on a solid frame between two opposite curved arms of this frame so that they are parallel to a first axis Y, corresponding to the height of the screen, which has a smaller dimension than the said screen. The strips are rectilinear and very highly tensioned on the frame along the first direction Y and the frame should be very solid to keep the mask under tension along this direction Y.
With the flat tension mask (FTM), the problem is different inasmuch as it has to be subjected to mechanical tension which is uniform in all directions.
There is a prior art method for mounting a flat tension mask on a frame to obtain mechanical tension of the mask along the first axis Y and along a second axis X perpendicular to the first axis. For this purpose, the prior art method consists in joining the periphery of the metallic flat tension mask to a glass frame by a welding operation in which the flat tension mask and the glass frame are heated to about 400.degree.. The flat mask is held on the glass frame by a removable tool while the assembly is being cooled. As the expansion coefficient of the metallic flat tension mask is greater than that of the glass frame, after the assembly is cooled, the flat mask is mounted in mechanical tension on the glass frame. One of the disadvantages of this method is that the glass frame is relatively brittle in itself and must have a cross-section which is large enough to give it the mechanical sturdiness needed to bear the mechanical tension of the flat tension mask, and also to withstand any shocks which may occur during the many subsequent operations for handling the frame/flat mask assembly. Consequently, the frame is very bulky and this considerably complicates its mounting in the tube. In this prior art method, this mounting is done by welding the frame on one side of the rear of the envelope and by welding it on the other side against the flared end of the glass forming the tube. The frame thus forms a part of the tube wall between the envelope and the glass of the tube.
Another disadvantage of this mounting is that it also complicates the operations for positioning the frame/mask to make the screen. For the positioning means that are then used should make it possible to place the frame/mask assembly in the same position as the one that will be occupied by this assembly when it is finally fixed. Now, since the final fixing of the assembly is done by welding, it is seen that the means used for the positioning and final fixing of the frame/mask assembly are not the same as those used to position and hold this assembly to make the screen. It must be further noted that the final assembly of the frame/mask assembly requires the use of very complicated, special-purpose tools in a complicated and expensive operation during which the entire tube and these tools are placed in a furnace to fix the frame by welding at a temperature of more than 400.degree.. Furthermore, in this frame welding operation, the mask itself is heated to a high temperature so that it is again expanded as it was during its mounting on the frame, so that there is a risk of variation in the mechanical tension of the mask and a risk of variation in its position with respect to the screen.